The invention relates to a method and an apparatus for improving the voltage quality of a secondary power supply unit through the use of a compensation device which has a pulse-controlled power converter with at least one capacitive storage device, a matching filter, a closed-loop and open-loop control device and an incoming feeder device, the compensation device being coupled serially to the power system through the use of a transformer.
Such a compensation device is known from an article entitled xe2x80x9cNetzqualitxc3xa4t im Griff [Tackling Power System Quality]xe2x80x9d in the journal xe2x80x9cEV Report-Information des Bereichs Energiexc3xcbertragung und -verteilung [Electronic Processing Reportxe2x80x94Information on Power Transmission and Distribution]xe2x80x9d, from the firm Siemens, pages 16 to 18, 1996, Order No. E50001-U700-R964. That compensation device, which is also referred to as SIPCON S, is connected directly into the load flow. Through the use of that compensation device, an additional voltage is added to the power system voltage and the supply voltage of a load is thus kept constant (secondary power supply unit). The energy which is fed-in in that device is taken from the voltage link to which power is continuously fed from the power system through the use of a diode rectifier as an incoming feeder device. An energy accumulator may also be provided as an incoming feeder device. Through the use of that compensation device, it is also possible to eliminate asymmetrical voltage dips or increases (1 or 2 pole faults) in the power system. It is necessary to make the incoming feeder device capable of feedback in order to compensate voltage increases. In addition, voltage distortions in the power system voltage which are generated by harmonics can be kept away from the power supply voltage of a load with that compensation device.
That paper also states that a pulse-width-modulated IGBT power converter, which has a d.c. voltage capacitor, is provided as a pulse-controlled power converter of that compensation device. The connection to the power system is made through the use of a matching filter, for example an LCL combination. The method of coupling the compensation device determines its method of operation. The serial method of coupling optimizes the voltage quality which is supplied to a load from the outside. In contrast, a parallel method of coupling cleans up the currents which go from a load into a power system. Correspondingly, the compensation device with serial coupling corresponds to a controlled voltage source, whereas the compensation device with parallel coupling corresponds to a controlled power source.
Voltage changes in a power supply system arise, for example, due to power system faults or switching operations. The changes can leave the permitted voltage range and thus lead to a failure of loads (for example a voltage dip to 50% of the rated value causes a contactor to be dropped or a rotational-speed-variable drive to switch off) or even to loads being destroyed (20% overvoltage). Therefore, for fault-free operation it is necessary to compensate those changes in the power system voltage. Studies have shown that the most frequent cause of voltage dips are faults in the transmission and distribution power system. The period of time until a fault is detected can extend between a few cycles and a few seconds. Those voltage dips (for example below 70% for a few cycles) can disrupt automated processes because the functioning of computers, robots and drives depends heavily on the voltage quality.
The increasing use of nonlinear loads (in particular diode rectifiers such as are located, for example, in power supply units of PCs, television sets, etc.) in power supply systems distorts the voltage increasingly. Their currents have, in fact, high harmonic levels and cause voltage drops across the power system impedances which are superimposed on the originally sinusoidal power system voltage. At excessively high values, those voltage distortions can lead to overloading of the power system operating equipment (e.g. transformers, compensation systems) and disrupt the orderly operation of other loads.
Public power companies and national working groups (for example IEC) have therefore issued recommendations relating to the maximum permissible voltage distortion which a load may cause. Those recommendations have been used as a basis for the EN standards which came into force in January 1996. So-called compatibility levels for individual harmonics in low-voltage power system have been defined, for example. Equipment manufacturers must develop their products in such a way that equipment can still function without faults with those distortion values. Power companies must ensure that the compatibility levels are not exceeded in their power system.
However, in many power systems the power system voltage distortion has already reached the compatibility level and a further increase is expected. For that reason, it is important to protect sensitive equipment against harmonics present in the power system voltage. That problem also includes the undesired filtering out of a ripple control signal into secondary power supply units.
Heretofore the problem of power system supply harmonics and of the filtering out of a ripple control signal has been solved by using conventional blocking filter circuits. Since the middle of the 80s, active filters have also been used which have control methods that operate both in the time domain and frequency domain. In the conference report entitled xe2x80x9cNew Trends in Active Filtersxe2x80x9d by H. Akagi, reprinted in xe2x80x9cConference Proceedings of EPE ""95xe2x80x9d in Seville, pages 0.017 to 0.026, various active filters have been proposed.
An ideal, three-phase power supply system supplies the load with three purely sinusoidal voltages which have a constant frequency and are displaced by 120xc2x0 el. with respect to one another and have constant, identical peak values, i.e. a pure positive phase-sequence system space-vector with a rated voltage as amplitude. The ideal power system currents for that power system are proportional to the corresponding conductor/ground power system voltage in each phase. The proportionality factor is equal in all three phases and is constant with steady-state loads. That is because a required quantity of energy or active power is then transferred with the minimum collective current r.m.s value and thus with the lowest-possible capacity utilization of the power system. Those currents are defined as active currents. Such an ideal load displays a steady-state characteristic for the power supply system, like a three-phase balanced resistive impedance.
Any load which deviates from that characteristic produces current components which contribute nothing to the transmission of active power. Those are referred to as reactive currents. Assuming that the power supply voltages approximately correspond to the above-mentioned ideal case, those reactive currents contain the harmonic currents (including a d.c. component), the frequency of which is a multiple of the power system frequency, the fundamental displacement reactive currents, which are produced by the phase displacement between the power system voltage fundamental and power system current fundamental, and the fundamental negative phase-sequence system currents which are due to asymmetrical loads. The harmonic currents are generally divided into harmonics (harmonic frequency as an integral multiple of the power system frequency), interharmonics (harmonic frequency as a rational multiple of the power system frequency) and quasi-harmonics (harmonic frequency as an irrational multiple of the power system frequency).
Those reactive current components give rise to an undesired voltage drop at the power system impedances and cause distorted power system voltages for other loads. Likewise, loads which are switched statistically (nonperiodically) or power system errors give rise to distorted voltages for other loads.
Generally, the power system voltage is composed of the required fundamental positive phase-sequence system component with rated value amplitude and the distortion components. Those distortion components of the power system voltage can be distinguished as follows:
1. harmonic components in the wider sense (harmonics, interharmonics, quasiharmonics)
2. fundamental negative phase-sequence system
3. difference between the amplitude of the fundamental positive phase-sequence system and the rated value.
It is accordingly an object of the invention to provide a method and an apparatus for improving the voltage quality of a secondary power supply unit, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and apparatuses of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for improving the voltage quality of a secondary power supply unit, which comprises providing a compensation device having a pulse-controlled power converter with at least one capacitive storage device, a matching filter, a closed-loop and open-loop control device and an incoming feeder device; serially coupling the compensation device to a power system with a coupling transformer; determining a fundamental positive phase-sequence system deviation of a determined power system voltage space-vector as a function of a predefined positive phase-sequence system setpoint voltage; determining a fundamental negative phase-sequence system deviation of the determined power system voltage space-vector as a function of a predetermined fundamental negative phase-sequence system setpoint-space-vector; determining a basic transmission ratio space-vector as a function of the determined fundamental positive and negative phase-sequence system deviations, of a transformation transmission ratio of the coupling transformer and of a value of a link voltage of the pulse-controlled power converter; and generating control signals for the pulse-controlled power converter of the compensation device as a function of the determined basic transmission ratio space-vector of the link voltage of the pulse-controlled power converter.
In order to improve the voltage quality of a secondary power supply unit, distorted power system voltage components must be kept away from this secondary power supply unit, for example a load. For this purpose, the compensation device must feed in these components serially through the use of a transformer between the power system and the load. For this purpose the nonideal voltage components to be compensated for are initially identified from a power system voltage space-vector. From these identified undesired voltage components, at least one basic transmission ratio space-vector is calculated, through the use of which a corresponding compensation voltage space-vector is then generated at the output of the pulse-controlled power converter of the compensation device. This compensation voltage space-vector is used to change a power system voltage space-vector with unwanted voltage components into a power system voltage setpoint-space-vector.
A publication entitled xe2x80x9cShunt-Connected Power Conditioner for Improvement of Power Quality in Distribution Networksxe2x80x9d, reprinted in xe2x80x9cInternational Conference on Harmonics and Quality of Powerxe2x80x9d, Las Vegas, Oct. 16-18, 1996, discloses a control method for a compensation device with parallel coupling. That conference report shows that the compensation voltage space-vector is calculated from the voltage drop across the capacitive storage device and from a transmission characteristic space-vector. In addition, that report indicates that the transmission ratio space-vector can be composed of a plurality of partial-ratio space-vectors. Additionally, it is shown how the partial transmission ratio space-vectors are determined. As mentioned at the outset, a compensation device with parallel coupling behaves like a controlled current source and a compensation device with serial coupling behaves like a controlled voltage source. The control characteristics for that known compensation device can therefore not be applied to a compensating device with serial coupling.
The aforesaid distortion components of the power system voltage can be eliminated individually or in any desired combination with one another. In order to ensure that the power system voltage space-vector has only one positive phase-sequence system space-vector of the secondary power supply unit with rated voltage as amplitude (ideal power supply system), at least one corresponding basic transmission ratio space-vector has to be generated.
In accordance with another mode of the invention, individual harmonics of the positive and/or negative phase-sequence system are eliminated by determining corresponding partial transmission ratio space-vectors, and the space-vectors are then added to the basic transmission ratio space-vector.
In accordance with a further mode of the invention, a partial transmission ratio space-vector for an active power transfer is acquired and superimposed at least on the basic transmission ratio space-vector. Thus, not only are unwanted voltage components kept away from the primary power supply unit but this also gives rise to an active power exchange and thus causes the link voltage of the pulse-controlled power converter to be regulated.
In accordance with an added mode of the invention, a correction value which is determined as a function of a determined reactive displacement power of the fundamental and a constant is added to the positive phase-sequence system setpoint voltage. This compensates the voltage drop across the coupling filter and across the transformer, which is caused by a load current component of the fundamental positive phase-sequence system.
With the objects of the invention in view, there is also provided an apparatus for improving the voltage quality of a secondary power supply unit, comprising a compensation device having a pulse-controlled power converter with at least one capacitive storage device, a matching filter, a closed-loop and open-loop control device and an incoming feeder device; the closed-loop and open-loop control device having a closed-loop control device for determining a transmission ratio space-vector, and the closed-loop and open-loop control device having a pulse-width modulator with outputs supplying control signals for the pulse-controlled power converter; a coupling transformer serially coupling the compensator device to a power supply system; the closed-loop control device having a controller for determining a basic transmission ratio space-vector, the controller having an input side, a positive phase-sequence system channel and a negative phase-sequence system channel on the input side, the positive phase-sequence system channel and the negative phase-sequence system channel having output sides, and an output-side computing device having inputs connected to the output sides of the positive phase-sequence system channel and the negative phase-sequence system channel; the computing device having an output supplying the basic transmission ratio space-vector, and the computing device receiving a value of a link voltage of the capacitive storage device of the pulse-controlled power converter and a value of a transformer transmission ratio of the coupling transformer; and the positive phase-sequence system channel and the negative phase-sequence system channel each receiving a determined power system voltage space-vector and each having an output supplying a respective one of a fundamental positive phase-sequence system deviation and a fundamental negative phase-sequence system deviation.
Through the use of these two channels, a positive phase-sequence system deviation and a negative phase-sequence system deviation are determined. The basic transmission ratio space-vector is then determined from the deviation values as a function of the link voltage of the pulse-controlled power converter and of a transformer transmission ratio using the computing device. Through the use of this basic transmission ratio space-vector, the pulse-controlled power converter generates a compensator voltage space-vector, as a result of which the power system voltage space-vector in the secondary power supply unit is now only a positive phase-sequence system space-vector with the rated voltage as amplitude. The determined positive and negative phase-sequence system deviations are each a measure of the distortion components which are present in the power system voltage and which are kept away from the secondary power supply unit.
In accordance with another feature of the invention, the transmission ratio space-vector is composed of the basic transmission ratio space-vector and at least one partial transmission ratio space-vector through the use of which harmonics of the positive and negative phase-sequence systems can be compensated. That is to say, for example, three further controllers are needed to compensate the distortion voltage components of the fundamental negative phase-sequence system, the 5th harmonic of the negative phase-sequence system and the 7th harmonic of the positive phase-sequence system of the power system voltage. Each controller calculates, from a determined voltage space-vector, for example a load-voltage space-vector, a partial transmission ratio space-vector which is added to an overall transmission ratio space-vector.
In accordance with a further feature of the invention, the fundamental negative phase-sequence system actual-space-vector is determined from the difference between the power system voltage space-vector and an identified fundamental positive phase-sequence system actual-space-vector. Since the power system voltage space-vector is used to acquire this vector without any further operation, the fundamental actual-space-vector is obtained directly without time delay. As a result, this device for acquiring a basic transmission ratio space-vector has very high dynamics. However, as a result of the formation of differences, not only the fundamental negative phase-sequence system actual-space-vector but also harmonics of the power system voltage which may possibly be present, are obtained. If the fundamental negative phase-sequence system actual-space-vector is identified, in the same way as the fundamental positive phase-sequence system actual-space-vector, from the power system voltage space-vector, this device for acquiring a basic transmission ratio space-vector will experience a decisive loss of dynamic response.
In accordance with a concomitant feature of the invention, there is provided a device for acquiring a partial transmission ratio space-vector for an active power transfer in the closed-loop control device of the pulse-controlled power converter. This device brings about an active power exchange, as a result of which the link voltage of the pulse-controlled power converter is regulated to a setpoint value. For this purpose, this device is supplied with a compensator current space-vector from which a frequency system, for example the fundamental positive phase-sequence system, is identified through the use of a discrete Fourier transformation and an inverse discrete Fourier transformation. This identified space-vector is then multiplied by a manipulated variable of a link voltage control loop. The partial transmission ratio space-vector which is obtained in this way is added at least to the basic transmission ratio space-vector to form an overall transmission ratio space-vector.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and an apparatus for improving the voltage quality of a secondary power supply unit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.